Examples of such two-regime CVT's of the toroidal-race rolling-traction type are to be found described in Patent Specification GB-A-1078791, GB-B-2150240 and many other prior publications. It has been a common feature of most such known CVT's that in the first or low regime, the gearing combination typically --of epicyclic type--has been used in conjunction with the variator as an output side or downstream shunt: in this regime power has recirculated, and the variator output has subtracted speed from its engine-driven input. In contrast in the second or high regime, there has in effect been a direct driving connection between the engine and the CVT output: the variator has taken full engine power, and the output range of the transmission has been the same as that of the variator.
In a variator which reverses the direction of rotation between input and output, the torque reaction (on the casing) will be the sum of the input and output torques. The tractive forces between the discs and rollers are proportional to the torque reaction. The ratio between tractive force and normal force between each disc and roller, commonly known in the art as the "design traction coefficient", is usually fixed by the layout of the variator. The normal force determines the contact stresses and therefore the size of the variator for a specified fatigue life. For any given operating condition size is approximately proportional to the cube root of the normal force, i.e. the cube root of the torque reaction. It is therefore advantageous to reduce torque reactions as far as possible.
If, for example, a variator with a ratio range from 0.5:1 to 2.0:1 is connected in high ratio to an input producing 100 Nm of torque, the torque reactions would range from 300 Nm at the low output speed end of the range to 150 Nm at the high end. The size of a variator will be influenced by its duty cycle, i.e. the time spent at full and part load and the time spent at each ratio. In most road vehicles little time is spent in low regime. Hard acceleration or hill climbing in high regime will tend to utilize low variator ratios at large engine torques. Benefits in size and weight (size cubed) can therefore be achieved by reducing the maximum torque reaction which occurs in high regime at the low end of the variator ratio range.